This invention relates to microwave cooking containers.
Microwave ovens are essentially heaters employing a magnitron which radiates electromagnetic energy in short radio frequencies. The energy waves are distributed randomly throughout the oven and are reflected and re-reflected from its inner surfaces inwardly until absorbed by the product being heated. Such repeated reflections and the interference between waves results in some energy loss.
Prior art microwave cooking containers are generally transparent to microwave energy to permit the interior of the food product to be heated through direct microwave absorption. This stimulates high frequency oscillations that cause molecular friction to generate heat. Such direct heating of high moisture food products is limited to about 100.degree. C. at which point, the water content begins to vaporize. Upon reaching vaporization, such water molecules move through the product and disburse in the ambient atmosphere where a portion condenses on the surface of the food product. This precludes browning or crisping and makes the food product soggy.
In order to elevate the cooking temperature above 100.degree. C., some prior art microwave cooking containers are provided with a microwave energy absorbing material which are commonly referred to as susceptors. These materials or susceptors are generally located in the lower end of the package and heat the food product by conduction and radiation.
Prior art microwave cooking containers having susceptors were not wholly satisfactory because they tended to heat unevenly resulting in hot spots that can cause the container and/or the food product to overheat. This resulted in uneven heating and sometimes caused burning of the susceptor which imparted a bad flavor to the food and inconsistent or uneven heating. Moreover, such overheating can dry out or overcook the food product in random, spotty zones. Also, because the heat is transferred from the susceptor to the food product by conduction as well as radiation, the food product, in effect, acts as a heat sink for the susceptor. As a result, the food product must be placed in close contact with the susceptor to avoid localized overheating. This is a particular problem when cooking food products which are irregularly shaped or are non-uniform in density. Heated susceptors also presented a safety hazard to users requiring some degree of caution.
One type of prior art microwave cooking container comprises a bag or pouch containing unpopped popcorn and hydrogenated cooking fat. A susceptor in the bottom of the bag or pouch becomes heated as it absorbs microwave radiation. The heat is conducted upwardly into the popcorn kernels. As the corn heats, water stored in the kernels vaporizes causing the internal pressure to rise until the outer shell or pericarp of the kernel ruptures explosively. The bag or pouch is constructed to permit expansion under the influence of the internal vapor pressure to accommodate the increase in volume as the corn pops.
The expanding bag or pouch also provides relatively nonplanar surfaces which do not readily reflect the microwave radiation within the oven. Until the microwaves are absorbed by the susceptor or the corn to be popped, they are reflected from the internal surfaces of the oven and collide with other reflective rays thereby losing some of their energy. This delays the cooking time and permits the corn kernels to lose some of their heat between impacts of microwave radiation. Also, because the corn is heated primarily from below by the susceptor, localized hot spots are created which results in a high proportion of unpopped kernels.
Also, because the heat absorbed by a body is directly related to its size, only the largest size kernels were employed for microwave popcorn. This limited the varieties of popcorn which could be employed.
Cooking oil accounts for about 97% of the fat content in microwave popcorn. Because expandable prior art microwave popcorn containers were sealed, it was not possible for the consumer to reduce the fat or salt content. This made the product unsuitable for persons on low fat or low salt diets. Such sealed packages were also unsatisfactory because they did not permit steam to escape so that the popcorn tended to be soggy.